01 - Preheat your oven to 400°F. Lightly grease a baking dish with cooking spray or a small amount of olive oil.
02 - Place each chicken breast on a cutting board. Using a sharp knife, slice lengthwise through the thickest part of the breast, being careful not to cut all the way through. Open the breast like a book to create a thin, even layer.
03 - Season both sides of each butterflied chicken breast evenly with 1/2 teaspoon salt and 1/2 teaspoon black pepper, rubbing the seasoning into the meat.
04 - In a medium mixing bowl, combine ricotta cheese, chopped spinach, 1/3 cup parmesan cheese, mozzarella cheese, minced garlic, nutmeg, 1/4 teaspoon salt, and 1/4 teaspoon black pepper. Mix thoroughly until all ingredients are evenly incorporated.
05 - Divide the spinach ricotta mixture evenly among the 4 chicken breasts, spreading it over one half of each butterflied breast. Fold the plain side over the filling to enclose completely.
06 - Secure the edges of each chicken breast with toothpicks if needed to prevent filling from escaping. Place stuffed breasts in the prepared baking dish, seam side down.
07 - Brush the tops of each chicken breast with 1 tablespoon olive oil. Sprinkle the remaining 1/4 cup grated parmesan cheese evenly over all breasts.
08 - Bake for 25 to 30 minutes, or until chicken is cooked through and reaches an internal temperature of 165°F when tested with a meat thermometer. The tops should be golden brown and the cheese melted.
09 - Remove from oven and let the chicken rest for 5 minutes to allow juices to redistribute. Carefully remove toothpicks before slicing crosswise and serving warm.